When a stream of pulse amplitude modulate (PAM) signals is transmitted through a serial point-to-point communication medium with passive high pass filters in the form of transformers or any high pass filter, the received signal at a receiver will be DC free as the high pass filters attenuate the low frequency components of the PAM signal. If a stream of the PAM signals is not DC free, then the DC of the stream begins to shift towards either positive voltage or negative voltage from the nominal. This phenomenon is called baseline wander (BLW), which occurs normally due to the high pass nature of the network transformers or any form of high pass filter and the symbols used for data transmission without frequent positive and negative voltage transitions. When BLW occurs, the bit error rate (BER) performance of a system deteriorates. The most common solutions to improve the baseline wander phenomenon are: 1) compensating the BLW by regenerating the approximate DC value of the transmitted signal in the local receiver, 2) compensating the BLW by estimating an equivalent DC value of the transmitting signal in the local transmitter before sending a stream of PAM signals (this may not result effective since it is a feed forward approach), and 3) using a bit-to-symbol mapping technique with sufficient positive and negative voltage transitions of symbols at the transmitter.
For the first solution, conventional techniques use a baseline wander compensating unit, which is also called a baseline wander circuit, at a receiver that compensates for the change in DC level of the received signal. Using this solution, the compensation of the BLW is done effectively in the digital domain using a digital signal processing (DSP) algorithm. Thus, the use of a baseline wander circuit in a high speed serial communication system increases the cost in terms of silicon area and power consumption of the receiver circuit.
For the second solution, the approach is based on feed forward, and thus, it may not result as an effective solution.
For the third solution, conventional bit-to-symbol mapping techniques do provide sufficient positive and negative voltage transitions of symbols at the transmitter to increase the BER performance of the system. However, there is a need for a bit-to-symbol mapping technique to further increase the BER performance of a serial communication system.
A device and method for encoding bits to symbols for a communication system are described. In one embodiment, a method for encoding bits to symbols for a communication system includes receiving a set of N-bit data to be transmitted, where N is an integer, generating side scrambling values using a polynomial, scrambling the set of N-bit data using the side scrambling values to produce scrambled data, mapping the scrambled data to a particular set of M symbols from a plurality of sets of M symbols, where M is an integer and M is smaller than N, and outputting the particular set of M symbols for transmission over a transmission medium. Other embodiments are also described.